1. Field of the Invention
The present invention relates to an ion source for generating negatively charged ions, comprising a closure plate, with an outlet opening provided therein, a wall surrounding a combustion chamber, wherein the wall has a tubular section, which extends from the outlet opening and is formed from an insulator material, and a rear wall, wherein the rear wall is arranged at that end of the tubular section which lies opposite the outlet opening, and closes off the combustion chamber, a coupling coil, the windings of which are arranged around the tubular section of the wall outside the combustion chamber, and a filter field magnet, arranged adjacent to the outlet opening, wherein the filter field magnet is configured in such a way that the field lines of the filter field magnet run transversely with respect to the extension direction of the tubular section.
2. Discussion of the Prior Art
Ion sources are generally employed whenever atoms are intended to be accelerated with the aid of electromagnetic fields. This is the case firstly in accelerator installations for scientific purposes, where high-energy ions are caused to collide with one another or with stationary targets in the field of nuclear and particle physics.
Secondly, in the field of semiconductor technology, materials are doped, that is to say impurity atoms are introduced into them, in such a way that the desired species of atoms is “implanted” into the material in the required amount. For this purpose, the corresponding atoms are shot into the material by means of an accelerator. This is associated with the advantage that the defined energy of the atoms can also be used to define the depth to which the atoms penetrate into the material.
In any case, it is necessary, however, for the atoms outwardly to be charged rather than neutral, in order that an electromagnetic field can exert an accelerating force on the atoms. It is necessary, therefore, for the atoms that are to be accelerated to be ionized, and thus for the electron shell not to contain the number of electrons which corresponds to the atomic number of the respective atom. Rather, the number of electrons must deviate from this in order that the charge of the atomic nucleus and that of the electron shell do not mutually compensate for one another.
In the simplest form of ionization, some of the electrons of the shell are removed from the respective atoms by a plasma in an ion source, such that the residual ions are then positively charged and can be extracted by a simultaneously applied electric field.
In specific accelerator installations, so-called tandem accelerators, in which the center of an acceleration path is at positive potential, it is necessary, however, firstly to generate negatively charged ions, which are then accelerated by the potential to the center of the acceleration path in order to be “stripped” there, that is to say to lose some of the electrons, and thus to be subjected to charge reversal from negative to positive ions. The potential, which now has the effect of repelling the ions, accelerates the latter again. In this case, the principle of the tandem accelerator is associated with the advantage that the acceleration potential is multiply utilized.
This means that it is necessary in this case to ensure in the ion source that the atoms have at least one electron more in their shell than is the case in the normal state. With regard to the ion source in tandem accelerators it should furthermore be taken into account that such an accelerator installation is generally set in such a way that only those ions which have a specific charge state are led further to the target after the acceleration process. However, the charge reversal is a statistical process and not all of the ions injected into the accelerator have the desired charge state after the charge reversal, rather a distribution arises which is dependent on the parameters during stripping.
This in turn has the effect that a considerable loss of beam current occurs as a result of the charge reversal, such that it is necessary to inject a very high beam current of negatively charged ions into the tandem accelerator in order consequently to obtain a sufficiently high current of positive ions having the desired charge state. This in turn presupposes that the ion source disposed upstream of the accelerator for generating negatively charged ions supplies a highest possible yield.
U.S. 2004/0104683, from which the present invention proceeds, discloses for this purpose an ion source comprising a closure plate with an outlet opening provided therein, and comprising a wall surrounding a combustion chamber, wherein the wall has a tubular section, which extends from the outlet opening and is formed from an insulator material, and a rear wall. The rear wall is arranged at that end of the tubular section which lies opposite the outlet opening, wherein the rear wall closes off the combustion chamber. In addition, a coupling coil is provided, the windings of which are arranged around the tubular section of the wall outside the combustion chamber in order to couple a radio-frequency field into the combustion chamber. Finally, a filter field magnet is provided, which is arranged adjacent to the outlet opening and serves to prevent energetic electrons from the plasma from leaving the combustion chamber through the outlet opening. Low-energy electrons can diffuse through the field, however, wherein the filter field magnet is configured in such a way that the field lines of the filter field magnet run transversely with respect to the extension direction of the tubular section. What is disadvantageous about an ion source of this type, however, is that the yield of negative ions is insufficient.
A further problem of this source is that the filter field magnet is arranged within the closure plate adjacent to the outlet opening and is thus exposed to the plasma. This can have the effect that the magnet incurs damage during operation of the source and the source has to be serviced after a short operating time. Moreover, said magnet influences the potential of the plasma in the outlet region by drawing electrons from the plasma.
Proceeding from the prior art, the object, therefore, is to provide an ion source for generating negatively charged ions which supplies an increased yield and is suitable for generating ions even from substances available only in gaseous form.